Rotating filter

ABSTRACT

A rotating filter having at least two annular filter disks each having a plurality of filter elements and being carried by a shaft for rotation about an axis perpendicular to the planes defined by the disks. A discharge chute extends into the space between the filter disks for discharging solids removed from the facing filter elements. An axial spacing between the first portion of the upper edges of the chute located radially inside radially inner edge portions of the facing filter elements of the adjacent filter disks is not less than an axial spacing between the inner edge portions, and an axial spacing between a second chute portion of the upper edges located radially outside the radially inner edge portions is not more than the axial spacing between the inner edge portions of the facing filter elements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application is the national phase under 35 U.S.C §371 of PCTInternational Application No. PCT/SE99/00081 which has an Internationalfiling date of Jan. 20, 1999, which designated the United States ofAmerica.

The present invention concerns a rotating filter, said filter includingat least two annular filter disks each having a plurality of filterelements and each being carried by a shaft for rotation therewith aboutan axis perpendicular to a respective central plane of the filter disks,said filter disks being partly submerged in a vessel adapted to containto a certain level a liquid to be filtered, and said filter furtherincluding discharge chute means extending into a space between adjacentfilter disks for discharging solids deposited on and removed from facingfiltering means of said filter elements.

2. Description of Background Art

WO 94/25140 (U.S. Pat. No. 5,656,162) describes a filter where thefilter elements are arranged at a relatively large distance from theshaft and have a relatively small radial extension, thereby giving thefilter disks an annular appearance Therefore, a filter of this kind isreferred to as a ring filter. In this prior art ring filter, thefiltering means of two adjacent filter rings have a common scraperarranged radially outside the filter rings. The tip end of the scraperintroduced between the filter rings is axially and radially guided bysliding in a U-shaped guide ring provided between the filter rings androtating therewith. The scraper also has a substantially U-shaped crosssection thereby to serve also as a chute for filter cake removed fromthe filtering means.

This prior art filter has proved to perform extremely well. However, incertain cases it has been found that, when increasing the radial heightof the filter elements, the amount or volume of filter cake materialdeposited on the filter elements becomes so large that, after itsremoved from the elements, the material may not be properly dischargedby the chute-like scraper running in the rotating, channel-shaped guidering between two adjacent filter rings. Thus, it may happen that aportion of a filter cake starts to rotate and twist between the scraperand the filtering means (normally a filter cloth) of the filter elementand to build up its dimension such that it eventually damages the filtercloth.

SUMMARY OF THE INVENTION

The present invention has its primary object to provide a rotatingfilter as initially stated, which allows a largely increased radialheight of the filter sectors in comparison to those according to theprior art referred to.

However, large radial heights of filter elements are well known in theart per se, see for instance U.S. Pat. No. 3,331,512. This patentdiscloses filter elements in the shape of sectors having a substantialradial extension from a shaft having axially extending filtratedischarge channels therein. To receive filter cake removed from twoadjacent filter disks, a chute is introduced into the space betweenthese disks. The chute extends from a position substantially straightabove and close to the shaft to a position exteriorly of the filterdisks. In a zone relatively close to the shaft, the chute is defined bylow side walls which substantially increase in height as the bottom ofthe chute steeply slopes towards the exterior of the disks, while thetop edges of the side walls extend in a substantially horizontaldirection to the exterior of the disks. This filter is evidentlyintended for handling substantial amounts of filter cake. However, thesame problem as referred to above will no doubt fall upon this filter,and that because of the obvious risk that portions of filter cakeremoved may get pinched between and edge of the chute and a surface of afilter sector, and there start rotating and building up a considerablethickness leading to damage of filter cloth, filter sectors and/orchute, and thereby to shutdown.

It is also an object of the present invention, thus, to provide arotating filter as initially stated which allows removal of substantialamounts of filter cake without any risks of portions thereof gettingtrapped between the discharge means (the chute) and a filter means of afilter sector.

According to the present invention, this is achieved in that the axialspacing between radially outer edge portion of a discharge chute locatedradially inside radially inner edge portions of facing filtering meansof adjacent filter disks is not less than the axial spacing between saidinner edge portions, and that the axial spacing between the radiallyouter edge portions of the discharge chute located radially outside saidradially inner edge portions is not more than said axial spacing betweensaid inner edge portions.

The term “axial” and “radial”, as well as forms derived therefrom, usedherein to define directions etc., refer to the rotational axis of theshaft of the filter and directions perpendicular to, or substantiallyperpendicular to, such axial directions.

Suitably, each filter element is radially spaced from said shaft byspacing means such that said radially inner edge portions of facingfiltering means of adjacent filter elements are radially spaced from theshaft, said spacing means having less axial extension than said radiallyinner edge portions thereby providing space for said radially outer edgeportions of said discharge chute to be located at a larger mutual axialspacing than said axial spacing between said radially inner edgeportions.

Advantageously, the spacing means comprise conduit means for dischargeof filtrate from the filter elements to said shaft.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

Embodiments of the present invention will now be described, referencebeing made to the accompanying drawings, wherein:

FIG. 1 is a top view of a typical filter embodying the presentinvention, an upper cover being removed;

FIG. 2 is a part-sectional view along line II—II of FIG. 1;

FIG. 3 is a sectional side view taken along line III—III of FIG. 1;

FIG. 4 is a cross section taken along line IV—IV of FIG. 2;

FIG. 5 is an enlarged portion of FIG. 4 particularly showing a crosssection of the shaft;

FIG. 6 is a perspective view of two diametrically opposed axial channelsand a portion of the shaft core;

FIG. 7 is an axial view of solely the chute of FIG. 4 showing more indetail the various features of a preferred embodiment;

FIG. 8 is a view of the chute of FIG. 7 as seen from the left in FIG. 7;

FIG. 9 is a view from above into the chute of FIG. 7 at an enlargedscale;

FIG. 10 is an enlarged sectional view through the preferred embodimentof the chute showing its relation to adjacent filter disks;

FIG. 11 is a view corresponding to that of FIG. 10 of a practicalembodiment of the chute according to FIG. 10;

FIG. 12 is a view corresponding to that of FIG. 10 of a secondembodiment of the chute; and

FIG. 13 is a view corresponding to that of FIG. 10 of a third embodimentof the chute.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The over-all structure of the filter will be briefly described withreference to FIGS. 1-4. A vessel 1 adapted to contain to a certain levela liquid to be filtered (hereinafter referred to as a suspension) haslongitudinal walls 2 and 3 end walls 4 and 5. Outside the latter arearranged bearings 6, 7, respectively, carrying opposed ends of a shaft8. The shaft, that is driveable for rotatable in a direction indicatedby an arrow A, carries a plurality of axially extending,circumferentially spaced filtrate channels 9 leading to a filtrate valve10 at the left end of the filter as seen in FIG 3. Each filtrate channel9 communicates with a plurality of axially spaced filtrate tubes 11.Each filtrate tube 11 extends in a radial direction from the respectivefiltrate channel 9. At its radially outer end, each filtrate tubecarries a filter sector 12, such that a plurality of filter sectorslocated in a common radial plane constitute a filter disk. Thus, in onefilter disk, each filter sector communicates through its own filtratetube with one of the axial filtrate channels, which is common to allfilter sectors having the same angular position relative to the shaft.

Each filter sector has axially opposed surfaces in the shape of a filtermedium, such as a filter cloth 13, to allow filtrate to enter into theinterior of the filter sector, are deposited on solid such as fibers ofa pulp suspension, are deposited on the surface to form a filter cake.The filtrate is withdrawn from the interior of the filter sector throughthe radial filter tube 11, into the axial filtrate channel 9 and throughthe filtrate valve 10, which communicates with a barometric leg 14 (onlythe upper end shown in FIGS. 2 and 3) creating the vacuum necessary forwithdrawal of filtrate.

Spray nozzles 15 are provided to promote initiating removal of filtercake by directing a jet of liquid, normally filtrate, towards radiallyouter portions of the filter disks 12 at rotational positions of thefilter sectors close to the top of each revolution. Oscillating sprayarms 16 are also provided for cleaning the filtering surfaces of thefilter sectors after removal of the filter cake. A chute 17 is insertedbetween adjacent filter disk to receive filter cake removed fromopposite surfaces 13 of adjacent disks. The chutes bring filter cakematerial to a discharge screw 18.

As is generally the case, the filter sectors taper towards their outercircumference symmetrically about a radial plane DC of each disk.

The arrangement of the filtrate tubes 11 as radially extending spacingmeans for the filter sectors results in an open structure allowing flowof suspension in an axial direction along the shaft and between thespoke-like filtrate tubes 11 from a centrally located inlet 19 in theend wall 4 of the vessel 1. Apart from the beneficial effect that thesuspension is firstly centrally fed and axially distributed through thevessel 1, and secondly allowed to flow radially outwards between thefilter disks, the rotating filtrate tubes 11 of each disk provide aconstant stirring of the suspension leading to a uniform and optimumdistribution thereof to all filter sectors submerged in the suspension.In this manner, thickening and concentration of solids in the vessel andat the outer circumference of the filter disks is eliminated. Thecentral and axial feed of suspension makes the customary inlet box at alongitudinal side of the vessel superfluous, which is advantageous,since such inlet box is expensive in manufacture and troublesome toservice and handle, particularly as regards control of the flowdistribution along the suspension vessel.

As appears particularly from FIG. 4, the filtrate tubes 11 are connectedto the filter sectors 12 at positions close to the trailing radial edgesof the sectors. It can further be seen, that the filtrate tubes 11 havea direction that is not truly radial, but substantially tangent to theperiphery of the shaft 8, thereby giving the tubes downwardly inclineddirections as the filter sectors raise above the level L of the liquidcontained in the vessel 1. Hereby is ensured that the filter sectors arerapidly drained and the filtrate directed into the axial filtratechannels 9 at a high flow rate. It can also be seen, that the filtratetubes 11 are upwardly inclined towards the shaft in rotational positionswhere the filter sectors are just submerged into the suspension. Thisresults in that filtrate entering the filter sectors effectivelydisplaces air from within the sectors through the upwardly incliningfiltrate tubes 11 and into the axial filtrate channels 9.

It also appears from FIG. 4 (combined with FIGS. 10-13) that thefiltrate tubes have a considerable length in relation to their crosssection. This involves the advantage that a filtrate tube will act as abarometric leg for its associated filter sector even before the filtratevalve has connected an associated axial filtrate channel to thebarometric leg 14, i.e., while cloudy filtrate is still dischargedthrough the filtrate channel. In the top position of a filter sector,the entire length of the filtrate tube will be added to the height ofthe barometric leg 14 and thereby further contribute to the suctioneffect.

The shape of the axial filtrate channels 9 appears particularly fromFIGS. 3, 5 and 6. It is seen in FIGS. 5 and 6 that the cross section ofthe channels is polygonal and widening from the distal end (which is theright hand end in FIG. 3) towards the proximal end which is the outletend communicating with the filtrate valve 10. More precisely, the shapeshown is quadrangular and includes a first, substantially radial channelside wall 20, which is supported by and attached to a tubular shaft core21, a channel bottom wall 22, a radial outer channel wall 23 and asecond substantially radially channel wall 24. The first channel sidewall 20 is constituted by a rectangular plate 25, the outer edge 26 ofwhich defines the outer radius of the combined shaft 8, i.e. the shaftcore 21 and the axial channels 9. The bottom wall 22 defines an anglewith the axis C such that diametrically opposed bottom walls convergetowards the outlet end of the channels (FIG. 3). The outer channel wall23 is constituted by a rectangular plate 27 having one edge 28 attachedto the radially outer edge 26 of the first side wall 20. In each outerchannel wall 23 is provided a plurality of mounting apertures 29 for thefiltrate tubes 11 of one axial row of such tubes. A radially outer edge30 of the second channel side wall 24 is connected to the rectangularplate 27 such that the effective outer wall 23 widens towards the outletend of the channel. Hereby, the second side walls incline relative toplanes through the axis C, such as the horizontal plane HP in FIG. 5,where the second side wall 24, being in that position the lowest portionof the channel, downwardly inclines towards the outlet end. Since thefirst side wall 20 as well as the second side wall 24 are connected tothe sloping bottom wall 22, it follows that both widen towards theoutlet end (see FIG. 3).

It is realized that the axial channels not only widen their crosssection towards the outlet end, but also that the lowest portions or“bottoms” of the channels slope towards the outlet end in rotationalpositions where filtrate is running through them. See for instance thechannel in positions about 3 o'clock, where the “bottom” is the secondside wall 24, the positions about 1 and 2 o'clock, where the second sidewall 24 and the bottom wall 22 constitute the “bottom”, and in positionabout 12 o'clock where the bottom wall 22 alone constitutes the“bottom”. This sloping features of the axial filtrate channelscontributes in an advantageous manner to the discharge of filtrate fromthe filter sectors, a.o. by contributing to the vacuum created by thebarometric leg 14 and the filtrate tubes 11.

Likewise, in about 180° opposite rotational positions, where the filtersectors are being lowered into the suspension, the second side wall 24and the bottom wall 22 slope upwards towards the outlet end and thefiltrate valve, thus contributing to an untroubled displacement of air.

In order to avoid the problem of bundles of filter cake being trappedbetween an upper edge of the chute and a surface of a filter sector, thepresent invention proposes to arrange the chutes such that upper edgesthereof, located vertically below locations of the filter sectors wherea major part of filter cake removal takes place, are positioned suchthat there is no vertical slot between such edges and a filter sectorsurface. In other words, the chute edges and the filter sector surfacesare at least located in the same vertical plane, and preferably thechute edges are displaced in an axial direction of the axis C, such thata chute edge is positioned in a radial plane located closer to a medianplane DC of a filter sector than a radial plane through a radially innerportion of a filter sector.

A preferred outline of a chute 17 according to the present invention isshown in FIG. 4, and particulars of the chute are shown more in detailin FIGS. 7-10. The chute has a flat bottom 31 including about 45° to ahorizontal plane and extending from radially outside the filter sectorsabove the discharge screw 18 to a position above the shaft that ispreferably located somewhat past a vertical plane VP (FIG. 4) throughthe axis C of the shaft. In the embodiment shown, the bottom 26 isrectangular. Axially spaced, substantially vertical side walls 32, 33extend perpendicularly from the bottom. Starting from the upper end ofthe bottom 31, the side walls 32, 33 have upper, radially outer edges34, 35, respectively, that are arcuately curved and have their center inthe axis C of the shaft. As is seen particularly in FIG. 4, the radiusof the curved edges 34, 35 is less than the radially inner edges 12′ ofthe filter seconds 12.

In the preferred embodiment of chute according to FIGS. 7-10, radiallyouter portion 36, 37 of the side walls 32, 33, including the curvededges 34, 35, are outwardly angled along a curved line 34′, 35′,respectively, co-axial to the edges 34, 35, so as to widen the open areaof the chute. This is best seen in FIG. 10, showing radially innerportions of three filter disks 12 a, 12 b and 12 c and particularly onecomplete chute 17 a located between adjacent disks 12 a and 12 b, andone complete chute 17 b located between adjacent disks 12 b and 12 c.

The radially inner edges of the disks are constituted by arcuatelycurved U-sections 38 having web portions 39 and spaced flanges 40, 41.In practice, each filter disk has a circularly shaped U-section as acommon support for all its filter sectors, the filtrate tubes 11 beingattached to the web portion 39 of the U-section as shown in FIG. 10.From the flanges 40, 41 each filter sector tapers radially outwards asseen particularly in FIGS. 1-3. Thus, the flanges form the widestportion of each filter sector.

In order to provide space for the outwardly inclined portions 36, 37 ofthe side walls and their outer edges 34, 35 radially inside the filterdisks, the filtrate tubes 11, radially extending in a planeperpendicular to the axis C, have less extension in the axial directionof the axis than the radially inner edges 12′ of the filter disks, i.e.,the web portion 39 in which the filtrate tubes 11 are attached. Thefiltrate tubes are attached to the U-sections 38 symmetrically about thecentral plane DC of each disk. Preferably, the filtrate tubes haverectangular cross sections, as shown in FIG. 10.

Evidently, due to the fact that the edges 34, 35 of the side walls ofthe chute are located “under” the U-section 38, i.e., the widest portionof a filter sector, filter cake material falling down along a filtersector will be securely caught within the chute without any risk ofentering the substantially axially extending, relatively small radialspace remaining between the edges 34, 35 and the web portion 39.

Apparently, it is not possible to have the upper edges of a chute shapedas now discussed along their entire extension towards the exterior ofthe disks, because the filter sectors have to pass the chute duringtheir rotation. Thus, the arcuate edges 34, 35 transform intosubstantially horizontally extending edges 42, 43 that upwardlyterminate outwardly bent portions 44, 45 of the side walls 32, 33. It isseen in FIGS. 7 and 9 that the outwardly bent portions 44, 45 are bentalong straight lines 44′ and 45′, thus gradually widening the upperedges 42, 43 of the chute to adapt it to tapering shape of the filteringdisks.

A practical embodiment of the chute according to FIG. 10 is shown inFIG. 11. In order to facilitate mounting of the chute between adjacentfilter disks, the angled, diverging portions of the side walls 32, 33are made as separately mountable and detachable elements 46, 47 havingmounting portions 48, 49 respectively, shown to be fastened to the sidewalls 32, 33 by means of screws and nuts 50, 51.

An alternative embodiment of chute 52 is shown in FIG. 12. This chutehas a wider bottom 53 and straight side walls 54, 55 that areperpendicular to the bottom. The width of the bottom is such that theupper edges 56, 57 of the side walls are located well beyond radialplanes defined by the flanges 40, 41 of the U-section 38 adjacent filtersector 12 a, 12 b and 12 b, 12 c, respectively.

Another embodiment of a chute is shown in FIG. 13. Here, the bottom 59has substantially the same width as the bottom 31 according to FIG. 10,i.e., less than the axial distance between flange portions 40, 41 of theU-sections 38 of adjacent filter sectors. The straight side walls 60, 61of the chute are perpendicular to the bottom. Thus the upper edges 62,63 of the side walls are not located “under” a respective inner edge ofa filter sector. In order to guide filter cake material falling from asector into the chute, the radially inner edges of the filter sectorsare provided with shield means leading filter cake material into thechutes. In practice, the flange portions 40, 41 of the U-sections 38 areprovided with diverging plates 64, 65 that straddle the upper edges 62,63 of adjacent chutes 58.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A rotating filter including at least two adjacentannular filter disks each having a plurality of filter elements (12 a/12b, 12 b/12 c) and each being carried by a shaft (8) for rotationtherewith about an axis (C) perpendicular to a respective central plane(DC) of the filter disks, said filter disks being partly submerged in avessel (1) adapted to contain to a certain level (L) a liquid to befiltered, and said filter further including a discharge chute (17; 52;57) extending into a space between said adjacent filter disks fordischarging solids deposited on and removed from facing filtering means(13) of said filter elements, said discharge chute having upper edges(34, 35; 56, 57; 61, 62), characterized in that an axial spacing betweena first portion (34, 35; 56, 57; 61, 62) of said upper edges locatedradially inside radially inner edge portion (12′) of facing filteringmeans of said adjacent filter disks is not less than axial spacingbetween said inner edge portions (12′), and that an axial spacingbetween a second portion (42, 43) of said upper edges located radiallyoutside said radially inner edge portions (12′) is not more than saidaxial spacing between said inner edge portions of facing filteringmeans.
 2. A filter according to claim 1, characterized in that eachfilter element (12) is spaced from said shaft by a spacing means (11)having less extension in an axial direction towards an adjacent filterdisc than a radially inner end (12′) of said filter element.
 3. A filteraccording to claim 2, characterized in that the spacing means is afiltrate tube (11) connecting a filter element (12) with a filtratechannel (9) of the shaft.
 4. A filter according to claim 2,characterized in that said first portion of said upper edges define anarc about said axis (C) having a first radius, and that said radiallyinner end (12′) of all filter elements (12) defines a circle having itscenter on said axis and a second radius larger than said first radius.5. A filter according to claim 1, characterized in that said upper edges(34, 35) of said first portion terminate diverging portions (36, 37; 46,47) of opposed side walls (32, 33) of said chute (17).
 6. A filteraccording to claim 5, characterized in that said diverging portions (46,47) are separately mountable and detachable side wall elements.
 7. Arotating filter including at least two annular filter disks each havinga plurality of filter elements (12) and each being carried by a shaft(8) for rotation therewith about an axis (C) perpendicular to respectivecentral plane (DC) of the filter disks, said filter disks beingsubmerged in a vessel (1) adapted to contain to a certain level (L) aliquid to be filtered, and said filter further including discharge means(17) extending into a space between adjacent filter disks for receivingand discharging solids deposited on and removed from facing filteringmeans of said adjacent filter elements, characterized by the combinationof the following features: a) each filter element (12) is kept at aradial distance from the shaft (8) by a tube means (11) for discharge offiltrate, said tube means having less extension in an axial directiontowards said adjacent filter element than a radially inner end (12′) ofsaid filter element; b) each means for discharging solids comprises achute (17) having a bottom portion (31) downwardly inclining from aposition above said shaft (8) to a position radially outside said filterdisks and having side walls (32, 33) extending from said bottom portion,said side walls having upper edges (34, 35) axially defining aneffective axial width of said chute as regards receiving solids removedfrom said facing filtering surfaces, said axial width being not lessthan an axial distance between radially inner ends (12′) of adjacentfilter sectors along a portion (34, 35) of said chute located radiallyinside said radially inner ends (12′) of filter sectors of said at leasttwo filter disks, and said axial width being less than said axialdistance between radially inner ends (12′) of adjacent filter sectorsalong a portion (42, 43) of said chute located radially outside saidinner ends of filter sectors of said at least two filter disks.